Monocytes and macrophages perform immunoregulatory, phagocytic, secretory, and tissue remodelling functions, which are vital in the body's defense against cancer. Originating in the marrow monocytes circulate in the blood, adhere to endothelium, before their diapedesis into normal tissues and areas of inflammation. They utilize an array of cytolytic mechanisms, which include tumor necrosis factor (TNF)/interleukin-1(IL-1), reactive oxidative intermediates, and phagocytosis. While being individually effective, these lytic pathways complement each other in killing tumors and organisms. In the past, research on macrophage differentiation and functions had been hindered by the lack of specific growth factors. The discovery of recombinant human monocyte-macrophage colony stimulating factor (rhM-CSF) has greatly facilitated this line of research. M-CSF is a lineage selective activator and growth factor. In vitro it induces monocytes to differentiate into macrophage-like cells, prolongs their survival, increases their mobility, and enhances their cytolytic potential. These activated monocytes also perform highly efficient antibody dependent cytotoxicity (ADCC), namely 100% kill at 1:1 E:T ratio) in the presence of monoclonal antibodies against a wide spectrum of human tumor cell lines. Phagocytosis is an early and important step in the lytic process, which is underestimated by standard radioisotope release assays. In vivo (rodents, cynomolgus monkeys, and humans) rhM-CSF induces monopoiesis in the marrow and monocytosis in the peripheral blood. The immunological and functional properties of these activated monocyte populations remain largely unexplored and their clinical potentials unknown. We propose to investigate the properties of monocytes isolated from patients undergoing rhM-CSF treatment in a phase I clinical trial. We want to define the rhM- CSF-induced changes in the expression of monocyte surface functional molecules (Fc receptors, adhesion molecules, cytokine receptors) which are vital to monocyte functions. We also propose to measure monocyte cytolytic capacity as reflected by their secreted and membrane bound TNF/IL-1, as well as by their reactive oxygen intermediates. We have developed and validated two assays (an enzyme-linked immunoadsorbent assay and a flow cytometric method) to measure phagocytosis of tumor cells. These in vitro measurements will provide an objective indicator of the biological response of human monocytes to intravenous rhM-CSF. These measurements will be correlated with the patients' clinical responses. We will estimate the optimal biological dose of rhM-CSF, a basis for future combination therapies.